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Versions: 00 01 02 03 04 05 06 07 rfc3560                               
S/MIME Working Group                                         R. Housley
Internet Draft                                                   SPYRUS
expires in six months                                     November 2000


          Use of the RSAES-OAEP Key Transport Algorithm in CMS

                <draft-ietf-smime-cms-rsaes-oaep-02.txt>


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
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Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes the use of the RSAES-OAEP key transport
   method of key management within the Cryptographic Message Syntax
   [CMS].

   This draft is being discussed on the "ietf-smime" mailing list.  To
   join the list, send a message to <ietf-smime-request@imc.org> with
   the single word "subscribe" in the body of the message.  Also, there
   is a Web site for the mailing list at <http://www.imc.org/ietf-
   smime/>.







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1  Introduction

   When the variant of the RSA key transport algorithm specified in PKCS
   #1 Version 1.5 [PKCS#1v1.5] is used for key management, it is
   vulnerable to adaptive chosen ciphertext attacks.  This attack is
   explained in [RSALAB] and [CRYPTO98].  The use of PKCS #1 Version 1.5
   key transport in interactive applications is especially vulnerable.
   Exploitation of this identified vulnerability, revealing the result
   of a particular RSA decryption, requires access to an oracle which
   will respond to hundreds of thousands of ciphertexts, which are
   constructed adaptively in response to previously-received replies
   providing information on the successes or failures of attempted
   decryption operations.

   The attack appears significantly less feasible in store-and-forward
   environments, such as S/MIME.  When PKCS #1 Version 1.5 key transport
   is applied as an intermediate encryption layer within an interactive
   request-response communications environment, exploitation could be
   more feasible.  However, Secure Sockets Layer (SSL) [SSL] and
   Transport Layer Security (TLS) [TLS] protocol implementations could
   include countermeasures that detect and prevent Bleichenbacher's and
   other chosen-ciphertext attacks, without changing the way the RSA key
   transport algorithm is used.  These countermeasures are performed
   within the protocol level.  In the interest of long-term security
   assurance, it is prudent to adopt an improved cryptographic technique
   rather than embedding countermeasures within protocols.

   An updated version of PKCS #1 has been published, PKCS #1 Version 2.0
   [PKCS#1v2.0].  This new document supersedes RFC 2313.  PKCS #1
   Version 2.0 preserves support for the encryption padding format
   defined in PKCS #1 Version 1.5 [PKCS#1v1.5], and it also defines a
   new alternative.  To resolve the adaptive chosen ciphertext
   vulnerability, the PKCS #1 Version 2.0 specifies and recommends use
   of Optimal Asymmetric Encryption Padding (OAEP) when RSA encryption
   is used to provide confidentiality, such as key transport.

   This document specifies the use of RSAES-OAEP key transport algorithm
   in the Cryptographic Message Syntax (CMS) [CMS].  CMS can be used in
   either a store-and-forward or an interactive request-response
   environment.

   CMS supports variety of architectures for certificate-based key
   management, particularly the one defined by the PKIX working group
   [PROFILE].  PKCS #1 Version 1.5 and PKCS #1 Version 2.0 require the
   same RSA public key information.  Thus, a certified RSA public key
   may be used with either RSA key transport technique.

   CMS values are generated using ASN.1 [X.208-88], using the Basic



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   Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules
   (DER) [X.509-88].

   Throughout this document, when the terms MUST, MUST NOT, SHOULD and
   MAY are used in capital letters, their use conforms to the
   definitions in [MUSTSHOULD].  [MUSTSHOULD] defines these key words to
   help make the intent of standards track documents as clear as
   possible.  The same key words are used in this document to help
   implementers achieve interoperability.  Implementations that claims
   compliance with this document MUST provide the capabilities as
   indicated by the MUST, MUST NOT, SHOULD and MAY terms.

2  Enveloped-data Conventions

   The CMS enveloped-data content type consists of encrypted content and
   wrapped content-encryption keys for one or more recipients.  The
   RSAES-OAEP key transport algorithm is used to wrap the content-
   encryption key for one recipient.

   Compliant software MUST meet the requirements for constructing an
   enveloped-data content type stated in [CMS] Section 6, "Enveloped-
   data Content Type".

   A content-encryption key MUST be randomly generated for each instance
   of an enveloped-data content type.  The content-encryption key is
   used to encipher the content.

2.1  EnvelopedData Fields

   The enveloped-data content type is ASN.1 encoded using the
   EnvelopedData syntax.  The fields of the EnvelopedData syntax must be
   populated as follows:

   The EnvelopedData version MUST be either 0 or 2.

   The EnvelopedData originatorInfo field is not used for the RSAES-OAEP
   key transport algorithm.  However, this field MAY be present to
   support recipients using other key management algorithms.

   The EnvelopedData recipientInfos CHOICE MUST be
   KeyTransRecipientInfo.  See section 2.2 for further discussion of
   KeyTransRecipientInfo.

   The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
   field MUST be specify a symmetric encryption algorithm.
   Implementations MUST support the encryption of Triple-DES content-
   encryption keys, but implementations MAY support other algorithms as
   well.



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   The EnvelopedData unprotectedAttrs MAY be present.

2.2  KeyTransRecipientInfo Fields

   The enveloped-data content type is ASN.1 encoded using the
   EnvelopedData syntax.  The fields of the EnvelopedData syntax must be
   populated as follows:


   The KeyTransRecipientInfo version MUST be either 0 or 2.  If the
   RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the
   version MUST be 0.  If the RecipientIdentifier is
   subjectKeyIdentifier, then the version MUST be 2.

   The KeyTransRecipientInfo RecipientIdentifier provides two
   alternatives for specifying the recipient's certificate, and thereby
   the recipient's public key.  The recipient's certificate must contain
   a RSA public key.  The content-encryption key is encrypted with the
   recipient's RSA public key.  The issuerAndSerialNumber alternative
   identifies the recipient's certificate by the issuer's distinguished
   name and the certificate serial number; the subjectKeyIdentifier
   identifies the recipient's certificate by the X.509
   subjectKeyIdentifier extension value.

   The KeyTransRecipientInfo keyEncryptionAlgorithm specifies that the
   RSAES-OAEP algorithm, and its associated parameters, was used to
   encrypt the content-encryption key for the recipient.  The key-
   encryption process is described in [PKCS#1v2.0].  See section 3 of
   this document for the algorithm identifier and the parameter syntax.

   The KeyTransRecipientInfo encryptedKey is the result of encrypting
   the content-encryption key in the recipient's RSA public key using
   the RSAES-OAEP algorithm. When using a Triple-DES content-encryption
   key, implementations MUST adjust the parity bits for each DES key
   comprising the Triple-DES key prior to RSAES-OAEP encryption.

3  RSAES-OAEP Algorithm Identifiers and Parameters

   The RSAES-OAEP key transport algorithm is the RSA encryption scheme
   defined in RFC 2437 [PKCS#1v2.0], where the message to be encrypted
   is the content-encryption key.  The algorithm identifier for RSAES-
   OAEP is:

      id-RSAES-OAEP  OBJECT IDENTIFIER  ::=  {
        iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 7  }

   The AlgorithmIdentifier parameters field must be present, and the
   parameters field must contain RSAES-OAEP-params.  RSAES-OAEP-params



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   have the following syntax:

      RSAES-OAEP-params  ::=  SEQUENCE  {
        hashFunc [0] AlgorithmIdentifier DEFAULT sha1Identifier,
        maskGenFunc [1] AlgorithmIdentifier DEFAULT mgf1SHA1Identifier,
        pSourceFunc [2] AlgorithmIdentifier DEFAULT pSpecifiedEmptyIdentifier  }

      sha1Identifier  ::=  AlgorithmIdentifier {
        id-sha1, NULL  }

      mgf1SHA1Identifier  ::=  AlgorithmIdentifier  {
        id-mgf1, sha1Identifier  }

      pSpecifiedEmptyIdentifier  ::=  AlgorithmIdentifier  {
        id-pSpecified, OCTET STRING SIZE (0)  }

      id-sha1  OBJECT IDENTIFIER ::=  {
        iso(1) identified-organization(3) oiw(14) secsig(3) algorithms(2) 26  }

      id-mgf1  OBJECT IDENTIFIER  ::=  {
        iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 8  }

      id-pSpecified  OBJECT IDENTIFIER  ::=  {
        iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 9  }

   The fields of type RSAES-OAEP-params have the following meanings:

      hashFunc identifies the one-way hash function.  Implementations
      MUST support SHA-1 [SHA1].  The SHA-1 algorithm identifier is
      comprised of the id-sha1 object identifier and a parameter of
      NULL.  Implementations that perform encryption MUST omit the
      hashFunc field when SHA-1 is used, indicating that the default
      algorithm was used.  Implementations that perform decryption MUST
      recognize both the id-sha1 object identifier and an absent
      hashFunc field as an indication that SHA-1 was used.

      maskGenFunc identifies the mask generation function.
      Implementations MUST support MFG1 [PKCS#1v2.0].  MFG1 requires a
      one-way hash function, and it is identified in the parameter field
      of the MFG1 algorithm identifier.  Implementations MUST support
      SHA-1 [SHA1].  The MFG1 algorithm identifier is comprised of the
      id-mgf1 object identifier and a parameter that contains the
      algorithm identifier of the one-way hash function employed with
      MFG1.  The SHA-1 algorithm identifier is comprised of the id-sha1
      object identifier and a parameter of NULL.  Implementations that
      perform encryption MUST omit the maskGenFunc field when MFG1 with
      SHA-1 is used, indicating that the default algorithm was used.
      Implementations that perform decryption MUST recognize both the



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      id-mgf1 and id-sha1 object identifiers as well as an absent
      maskGenFunc field as an indication that MFG1 with SHA-1 was used.

      pSourceFunc identifies the source (and possibly the value) of the
      encoding parameters, commonly called P.  Implementations MUST
      represent P by an algorithm identifier, id-pSpecified, indicating
      that P is explicitly provided as an OCTET STRING in the
      parameters.   The default value for P is an empty string.  In this
      case, pHash in EME-OAEP contains the hash of a zero length string.
      Implementations MUST support a zero length P value.
      Implementations that perform encryption MUST omit the pSourceFunc
      field when a zero length P value is used, indicating that the
      default value was used.  Implementations that perform decryption
      MUST recognize both the id-pSpecified object identifier and an
      absent pSourceFunc field as an indication that a zero length P
      value was used.

4  SMIMECapabilities Attribute Conventions

   RFC 2633, Section 2.5.2 defines the SMIMECapabilities signed
   attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be
   used to specify a partial list of algorithms that the software
   announcing the SMIMECapabilities can support.  When constructing a
   signedData object, compliant software MAY include the
   SMIMECapabilities signed attribute announcing that it supports the
   RSAES-OAEP algorithm.

   The SMIMECapability SEQUENCE representing RSAES-OAEP MUST include the
   id-RSAES-OAEP object identifier in the capabilityID field and MUST
   include the RSAES-OAEP-Default-Identifier SEQUENCE in the parameters
   field.

      RSAES-OAEP-Default-Identifier  ::=  AlgorithmIdentifier  {
        id-RSAES-OAEP,  {
          sha1Identifier, mgf1SHA1Identifier, pSpecifiedEmptyIdentifier  }  }

   When all of the default settings are selected, the SMIMECapability
   SEQUENCE representing RSAES-OAEP MUST be DER-encoded as:

      30 0D 06 09 2A 86 48 86 F7 0D 01 01 07 30 00

References

   CMS         Housley, R.  Cryptographic Message Syntax.  RFC 2630.
               June 1999.

   CRYPTO98    Bleichenbacher, D.  "Chosen Ciphertext Attacks Against
               Protocols Based on the RSA Encryption Standard PKCS #1,"



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               in H. Krawczyk (editor), Advances in Cryptology - CRYPTO '98
               Proceedings, Lecture Notes in Computer Science 1462 (1998),
               Springer-Verlag, pp. 1-12.

   MUSTSHOULD  Bradner, S.   Key Words for Use in RFCs to Indicate
               Requirement Levels.  BCP 14, RFC 2119.  March 1997.

   PKCS#1v1.5  Kaliski, B.  PKCS #1: RSA Encryption, Version 1.5.
               RFC 2313.  March 1998.

   PKCS#1v2.0  Kaliski, B.  PKCS #1: RSA Encryption, Version 2.0.
               RFC 2437.  October 1998.

   PROFILE     Housley, R., W. Ford, W. Polk, and D. Solo.  Internet
               X.509 Public Key Infrastructure: Certificate and CRL
               Profile.  RFC 2459.  January 1999.

   RANDOM      Eastlake, D., S. Crocker, and J. Schiller.  Randomness
               Recommendations for Security.  RFC 1750.  December 1994.

   RSALABS     Bleichenbacher, D., B. Kaliski, and J. Staddon.
               Recent Results on PKCS #1: RSA Encryption Standard.
               RSA Laboratories' Bulletin No. 7, June 26, 1998.
               [Available at http://www.rsasecurity.com/rsalabs/bulletins]

   SHA1        National Institute of Standards and Technology.
               FIPS Pub 180-1: Secure Hash Standard.  17 April 1995.

   SSL         Freier, A., P. Karlton, and P. Kocher.  The SSL Protocol,
               Version 3.0.  Netscape Communications.  November 1996.
               [Available at http://draft-freier-ssl-version3-02.txt]

   TLS         Dierks, T. and C. Allen.  The TLS Protocol Version 1.0.
               RFC 2246.  January 1999.

   X.208-88    CCITT.  Recommendation X.208: Specification of Abstract
               Syntax Notation One (ASN.1).  1988.

   X.209-88    CCITT.  Recommendation X.209: Specification of Basic Encoding
               Rules for Abstract Syntax Notation One (ASN.1).  1988.

   X.509-88    CCITT.  Recommendation X.509: The Directory - Authentication
               Framework.  1988.

Security Considerations

   Implementations must protect the RSA private key and the content-
   encryption key.  Compromise of the RSA private key may result in the



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   disclosure of all messages protected with that key.  Compromise of
   the content-encryption key may result in disclosure of the associated
   encrypted content.

   Implementations must protect the key management private key and the
   message-authentication key.  Compromise of the key management private
   key permits masquerade of authenticated data.  Compromise of the
   message-authentication key may result in undetectable modification of
   the authenticated content.

   The generation of RSA public/private key pairs relies on a random
   numbers.  The use of inadequate pseudo-random number generators
   (PRNGs) to generate cryptographic keys can result in little or no
   security.  An attacker may find it much easier to reproduce the PRNG
   environment that produced the keys, searching the resulting small set
   of possibilities, rather than brute force searching the whole key
   space.  The generation of quality random numbers is difficult.  RFC
   1750 [RANDOM] offers important guidance in this area.

Acknowledgments

   This document is the result of contributions from many professionals.
   I appreciate the hard work of all members of the IETF S/MIME Working
   Group.  I wish to extend a special thanks to Burt Kaliski.

Author Address

   Russell Housley
   SPYRUS
   381 Elden Street
   Suite 1120
   Herndon, VA 20170
   USA

   housley@spyrus.com



Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

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   included on all such copies and derivative works.  In addition, the



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   ASN.1 module presented in Appendix A may be used in whole or in part
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Housley                                                         [Page 9]